1. Field of the Invention
The present invention relates to a method of hardening gelatin using an improved hardening agent and particularly to a method of hardening gelatin for silver halide photographic light-sensitive materials.
2. Description of the Prior Art
Gelatin is used as binders for many kinds of photographic light-sensitive materials. For example, the gelatin is used as a main component for silver halide light-sensitive emulsion layers, emulsion protective layers, filter layers, intermediate layers, antihalation layers, backing layers, subbing layers of film bases or baryta layers, etc.
These light-sensitive materials containing the gelatin are processed with various kinds of aqueous solutions, each with a different pH or with a different temperature. The properties of layers containing gelatin which is not processed with a hardening agent depend mainly upon the properties of the gelatin, and such layers have a poor water resistance and the mechanical strength of such layers becomes very low due to excessive swelling in the aqueous solutions. Particularly, in extreme cases, the gelatin layers sometimes dissolve off into the solutions when aqueous solutions at a temperature higher than about 30.degree. C or highly alkaline aqueous solutions are used. These characteristics are fatal defects for layers in photographic light-sensitive materials.
It is known that many compounds are effective for hardening gelatin to improve the water resistance properties, heat resistance properties and scratch resistance properties of gelatin layers.
These compounds are well known as "hardening agents" used in production of photographic light-sensitive materials. For example, aldehyde compounds such as formaldehyde or glutaraldehyde, reactive halogen containing compounds described in U.S. Pat. No. 3,288,775, et al., compounds having ethylenically unsaturated reactive bonds described in U.S. Pat. No. 3,635,718, et al., aziridine compounds described in U.S. Pat. No. 3,017,280, epoxy compounds described in U.S. Pat. No. 3,091,537, halocarboxyaldehydes such as mucochloric acid, dioxanes such as dihydroxydioxane or dichlorodioxane, and inorganic hardening agents, such as chromium alum or zirconium sulfate, etc., are known as gelatin hardening agents.
However, all of these known compounds have some defects. Namely, some have an insufficient hardening function when used for photographic light-sensitive materials, some cause change to occur in quality with the lapse of time because of a hardening function called "post-hardening" which occurs due to a slow hardening reaction with gelatin, some compounds adversely influence the properties of the photographic light-sensitive materials (particularly, increase fogging and reduce sensitivity, etc.), some lose their hardening ability when certain photographic additives are present at the same time or result in a reduction in the functions of these other photographic additives (for example, couplers for color light-sensitive materials), some are difficult to synthesize in a large quantity and some have poor storability because they are unstable themselves.